The field and laboratory investigations needed to obtain the required data for the soils for proper design and successful construction of structure at the site are collectively known as soil exploration.
The selection of the foundation and its depth, the bearing capacity, settlement analysis depend very much upon the several engineering properties of the foundation soils.
The primary purposes of soil exploration:
- To determine the nature of the deposits of soil.
- To determine the depth and thickness of various soil strata and their extent in the horizontal direction.
- The area of the groundwater table (G WT).
- Collecting soil and rock samples from different strata.
- To determine the engineering properties of the soil and rock strata that affect the performance of the structure.
- To determine the in-situ properties by conducting field tests.
Required soil data:
a) Soil profile
- layer thickness and soil identification
b) Index properties
- Water content, Atterberg limits, etc.
c) Strength & compressibility characteristics
d) Others (c.g., water table depth)
Before the site investigation:
The first stage of site investigation required the following data if available
- Aerial photographs
- Topographical maps
- Existing site investigation reports(for nearby sites)
Methods for soil exploration :
- Direct: test pits, trial pits or trenches
- Semi-direct: Boring
- Indirect: Soundings or penetration tests and geophysical methods
Direct methods -Test pits
- Test pits or trenches are an open type or accessible soil exploratory system.
- Soils can be examined in their natural condition.
- Sampling procedures can collect the underlying soils samples and used for finding strength and other engineering properties by relevant laboratory tests.
- Test pits are considered fit only for small depths up to 3m; the cost of these increases quickly with depth.
- For greater depths, parallel supports or bracing of the excavations will be needed.
- Test pits are typically made only for improving other methods or for minor structures.
Semi-direct methods – boring :
a) Boring: Making or drilling boreholes into the ground with an outlook to obtaining soil or rock samples from particularised or known depths is known as boring.
b) The conventional methods of advancing boreholes:
- Auger boring
- Wash boring
- Rotary drilling
- Percussion drilling
Auger boring technique:
- Sol auger is a device that is useful for developing a borehole inside the ground.
- Augers may be hand-operated or power-driven; the first one is used for comparatively small depths (less than 3 to 5 m), while the second one is used for greater depths (up to 60 to 70 m in case of continuous-flight augers)
- The soil auger is driven by rotating it while pressing it into the soil.
- When the auger gets filled with soil, it is removed, and the soil sample is collected.
- The soil samples collected from this type of borings are very disturbed.
- Augur boring is possible in case of partially saturated sands, silts and medium to stiff cohesive soils.
Shell and auger:
- Used widely in India
- The shell, (called a sand bailer) is a heavy-duty pipe with a cutting edge.
- The shell is raised and let fall in a hole. The soil is cut, enters the tube, which is emptied when full.
- Shell is used when auger boring becomes difficult.
Wash boring technique:
- Wash boring is generally used for exploration below the groundwater table for which the auger method is not suitable.
- This method can be used in all kinds of soils without those mixed with gravel and boulders.
- A casing pipe is driven in and driven with a drop weight.
- A hollow drill bit is screwed into a hollow drill rod attached to a rope crossing over a pulley and held by a tripod.
- Water jet with pressure is applied through the rod and the bit into the hole. This loosens the soil at the deeper end and forces the soil-water suspension upwards along with the annular covering between the rod and the surface of the hole.
- This suspension is directed to a settling tank where the soil particles settle while the water drains into a sump. The water collected in the sump is reused for circulation for next time.
- The soil particles collected represent a very ‘disturbed sample’ and are not very useful for the evaluation of the engineering properties.
- Wash borings are primarily used for advancing boreholes; whenever a soil sample is needed, the chopping bit is to be substituted by a sampler.
- The change in the rate of progress and evolution of colour of wash water show changes in soil strata.
Rotary drilling technique:
- It can be used in sand, clay and rocks strata(unless badly fissured)
- A drill bit, fixed to the deeper end of a drill rod, which is rotated by power while being kept in firm touch with the hole.
- Drilling fluid or bentonite suspension is forced with pressure through the drill rod, and it comes upbringing the cuttings to the surface.
- Even rock cores may be achieved by using suitable diamond drill bits.
- When soil samples are required, the drilling rod raised G and drilling bit is replaced by a sampler.
Percussion drilling technique:
- A massive drill bitts suspended from a drilling rod or a cable and is driven by repeated blows. Water is added to facilitate the separating of hard soil or rock strata.
- The slurry of this material is bailed out at interims.
- This method cannot be used in unconfined sand and is slow in plastic clay.
- The formation gets severely disturbed by the impact.
Types of soil Samples:
- Disturbed sample: A disturbed sample is that in which the natural structure of the soil gets modified party or entirely during sampling.
- Undisturbed sample: An undisturbed sample is that in which the natural structure and other physical properties remain preserved.
Disturbed but representative samples can be used for:
- Grain-size analysis
- Determination of liquid and plastic limits
- The specific gravity of soil solids
- Organic content determination
- Soil classification
Undisturbed samples must be used for:
- Consolidation test
- Hydraulic conductivity test
- Shear strength test
- Required for triaxial, consolidation tests in the lab.
Degree of disturbance:
- Good quality samples are necessary.
- Thicker the wall, greater the disturbance.
- Take good care in transport and handling,
The degree of disturbance of a cohesive soil or rock sample can be estimated by recovery ratio.
Recovery ratio is equal to the actual length of the recovered sample divided by the theoretical length of the recovered sample.
- Recovery ratio = 1 (recovered length of the sample = the length sampler was forced into the stratum). Theoretically, the sample did not become compressed from friction on the tube. It indicates a functional recovery.
- Recovery ratio <1 indicates that the soil is compressed
- Recovery ratio >1 indicates that the sol has swelled
Types of Samplers:
- ‘Thick wall’ samplers (Split spoon sampler)
- ‘Thin wall’ samplers (Shelby tubes)
1. Split Spoon Sampler:
- A drive shoe attached to the deeper end works as the cutting edge. A sample cap may be screwed, at the highest end of split Spoon.
- The regular size of the spoon sampler is of 35 mm inner and 50.80 mm outer diameter.
- The sampler is dropped to the bottom of the borehole by connecting it to the drill rod. The sampler is then pushed by forcing it into the soil by blows from a hammer.
- The arrangement of the sampler is then removed from the hole, and the cutting edge and coupling at the upper end are unscrewed. The two halves of the barrel are departed, and the sample is thus revealed.
- Samples are generally taken at intervals of about 1.53m (5ft)
- When the material encountered in the filed is sand (particularly fine sand below the water table), a device known as a spring core catcher is installed inside the split spoon.
2. Thin-Walled Sampler:
- Commonly used to obtain undisturbed clayey samples. Outside diameter: 50.80 mm (2in) and 76.30 mm (3 in)
- Sampler with a 50.8 mm outside diameter has an inside diameter of about 47.63 mm. The area ratio is 13.75%
How many boreholes?
The number of boreholes depends on:
- type and size of the project
- budget for site investigation
- soil variability
Locate the boreholes where the loads are expected.
The spacing and depth of Borings:
Depending on the types of project spacing of boring may be varied
- Multistorey buildings: 10m-30m
- One-story industrial plants: 20m-60m
- Highways: 250m-500m
- Residential subdivision: 250m-500m
- Dams and dykes: 40m-80m
Minimum Depth of Boring (according to ASCE, 1972) for a building with, a breadth of 30.5 m (100 ft) will be as under
- Single storey: 3.50m
- Two storeys: 6.0m
- Three storeys: 10.0m
- Four storeys: 16.0m
- Five storeys: 24.0m
Depth of Borings (according to 1IS 1892-1979) for different types of foundation will be as under
- Isolated spread footing or raft foundation: One and half times the width (B) of the foundation.
- Adjacent footings with precise spacing less than twice the width: One and half times the length (L) of footing.
- Pile foundation and well foundation: To a depth of one and half times the width of structure from the level (toe of pile or bottom of bearing well)
- Road cut: Which is equal to the base width of the cut.
- Road Fill: Two meters below ground level or equal to the height of the fill whichever.
Ground Water Level:
A correct indication of the standard groundwater level is found by leaving the water in the boring to arrive at an equilibrium level.
- In sandy soils, the level becomes stabilized instantly within a few hours at the most.
- In clayey soils, it will take several days for this purpose. Hence, standpipes or piezometers used in clays and silt.